Aalborg Universitet. CFD in Ventilation Design Nielsen, Peter Vilhelm. Publication date: 2009

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1 Aalborg Universitet CFD in Ventilation Design Nielsen, Peter Vilhelm Publication date: 2009 Document Version Publisher's PDF, also known as Version of record Link to publication from Aalborg University Citation for published version (APA): Nielsen, P. V. (2009). CFD in Ventilation Design: a new REHVA Guide Book. Aalborg: Department of Civil Engineering, Aalborg University. General rights Copyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights.? Users may download and print one copy of any publication from the public portal for the purpose of private study or research.? You may not further distribute the material or use it for any profit-making activity or commercial gain? You may freely distribute the URL identifying the publication in the public portal? Take down policy If you believe that this document breaches copyright please contact us at vbn@aub.aau.dk providing details, and we will remove access to the work immediately and investigate your claim. Downloaded from vbn.aau.dk on: april 22, 2018

2 CFD in Ventilation Design, a new REHVA Guide Book by Peter V. Nielsen, Aalborg University Peter V. Nielsen, Aalborg University pvn@civil.auc.dk 1

3 Development in Computer Speed In D nonisothermal flow. 1 mio. grid points 39 CPU years In D nonisothermal flow. 1 mio. grid points A few CPU hours Peter V. Nielsen, Aalborg University pvn@civil.auc.dk 2

4 My First Experience with CFD Peter V. Nielsen, Aalborg University 3

5 The Development of CFD The computation cost is decreasing by a factor of 10 each eighth year. 7 x 10 31x31x31 10,000 Peter V. Nielsen, Aalborg University pvn@civil.auc.dk 4

6 CFD Prediction of Cross Infection Peter V. Nielsen, Aalborg University 5

7 The Amoy Gardens SARS Outbreak A A B B F E F D E C D C 6 Index patient visited Flats 7, 16 th Floor Index patient visited a flat 7 on two nights in March 2003 N Peter V. Nielsen, Aalborg University pvn@civil.auc.dk 6

8 Peter V. Nielsen, Aalborg University 7

9 Peter V. Nielsen, Aalborg University 8

10 Concentration Distribution with 45 degree Deflector Airflow rate with 100m 3 /h Airflow rate with 150m 3 /h Airflow rate with 200m 3 /h Peter V. Nielsen, Aalborg University pvn@civil.auc.dk 9

11 CFD in Ventilation Design Authors: Peter V. Nielsen (ed.), Francis Allard, Hazim B. Awbi, Lars Davidson and Alois Schälin Computational fluid dynamics in a nutshell Symbols and glossary Mathematical background Turbulence models Numerical methods Boundary conditions Quality control CFD combined with other prediction models Application of CFD codes in building design Case studies Benchmark tests Peter V. Nielsen, Aalborg University pvn@civil.auc.dk 10

12 About the design guide book The user of the design book is mainly considered to be a consulting engineer who has to: - order a CFD prediction - consider and work with a CFD prediction - discuss CFD and CFD quality with a supplier of a CFD prediction The book is in principle not written for engineers who are making CFD predictions Peter V. Nielsen, Aalborg University pvn@civil.auc.dk 11

13 CFD in Ventilation Design Computational fluid dynamics in a nutshell Symbols and glossary Mathematical background Turbulence models Numerical methods Boundary conditions Quality control CFD combined with other prediction models Application of CFD codes in building design Case studies Benchmark tests Peter V. Nielsen, Aalborg University pvn@civil.auc.dk 12

14 Peter V. Nielsen, Aalborg University 13 Mathematical Background From the general description Φ Φ + Φ Γ Φ = + Φ S grad div V div t ) ( ) ( ) ( ρ ρ to a two-dimensional time dependent transport equation + = y c x c D y c v x c u t c

15 CFD in Ventilation Design Computational fluid dynamics in a nutshell Symbols and glossary Mathematical background Turbulence models Numerical methods Boundary conditions Quality control CFD combined with other prediction models Application of CFD codes in building design Case studies Benchmark tests Peter V. Nielsen, Aalborg University pvn@civil.auc.dk 14

16 Turbulence Models 2D steady state laminar boundary layer flow/ turbulent boundary layer flow 2 c c c ρu + ρv =Γ c + S 2 x y y c A discussion of different turbulence models as the k-ε model, the k-ω model, the SST model and the Reynolds Stress model The Large Eddy Simulation is also discussed Peter V. Nielsen, Aalborg University pvn@civil.auc.dk 15

17 CFD in Ventilation Design Computational fluid dynamics in a nutshell Symbols and glossary Mathematical background Turbulence models Numerical methods Boundary conditions Quality control CFD combined with other prediction models Application of CFD codes in building design Case studies Benchmark tests Peter V. Nielsen, Aalborg University pvn@civil.auc.dk 16

18 One-Dimensional Case The case can be considered as a small part of a flow, which in certain areas is one-dimensional, parallel with grid lines and steady. dc d c ρu + dx dx 2 = Γc S 2 c Peter V. Nielsen, Aalborg University pvn@civil.auc.dk 17

19 Steady One-Dimensional Convection - Diffusion Transport Equation Peter V. Nielsen, Aalborg University pvn@civil.auc.dk 18 Analytical expression

20 One-Dimensional Discretization Equation Control-volume formulation (of mass fraction transport Equation in x direction): It is necessary to replace values at the cell surfaces e and w with values at the grid points WW, W, P, E and EE to have a final version of the discretization equation. Peter V. Nielsen, Aalborg University pvn@civil.auc.dk 19

21 Different Discretization Equations, 1 The flow is studied in a case where the length x is equal to 4. The boundary values c o and c 3 are equal to 1.0 and 0.0. Diffusion, u = 0.0 Peter V. Nielsen, Aalborg University pvn@civil.auc.dk 20

22 Different Discretization Equations, 2 The flow is studied in a case where the length x is equal to 4. The boundary values c o and c 3 are equal to 1.0 and 0.0. Central difference fx c e = (c P + c E )/2 and u = 0.1 Peter V. Nielsen, Aalborg University pvn@civil.auc.dk 21

23 Different Discretization Equations, 3 Central difference fx c e = (c P + c E )/2 and u = 3.0 xu Pe = ρδ Γ Wiggly for larger than 2 c Peter V. Nielsen, Aalborg University pvn@civil.auc.dk 22

24 History and Numerical Schemes, 1 The sixties The central difference scheme becomes unstable (wiggly) when the Peclet number is large. The cure is to decrease the grid size. The seventies Upwind difference opened the way for infinitely high Reynolds numbers, but false diffusion could in many cases be larger than diffusion of physical kind. The eighties and the nineties Second order schemes decreased the effect of false diffusion. Peter V. Nielsen, Aalborg University pvn@civil.auc.dk 23

25 Imperial College and the Upwind Scheme The upwind scheme was among others invented by Imperial College in the late sixties. Peter V. Nielsen, Aalborg University 24

26 Upwind Scheme Peter V. Nielsen, Aalborg University 25

27 Different Discretization Equations, 4 The flow is studied in a case where the length x is equal to 4. The boundary values c o and c 3 are equal to 1.0 and 0.0. Upwind scheme fx c e = c P and u = 3.0 Peter V. Nielsen, Aalborg University pvn@civil.auc.dk 26

28 History and Numerical Schemes, 2 The sixties The central difference scheme becomes unstable (wiggly) when the Peclet number is large. The cure is to decrease the grid size. The seventies Upwind difference opened the way for infinitely high Reynolds numbers, but false diffusion could in many cases be larger than diffusion of physical kind. The eighties and the nineties Second order schemes decreased the effect of false diffusion. Peter V. Nielsen, Aalborg University pvn@civil.auc.dk 27

29 False Diffusion Peter V. Nielsen, Aalborg University 28

30 Second-Order Upwind Scheme Second-order discretization error. Unbounded (e-value can be larger than W, P, E-values). Non-physical wiggles may occur. Peter V. Nielsen, Aalborg University 29

31 Higher Order Schemes Second order upwind scheme is an example of a new scheme developed in the middle of the seventies. The one-dimensional case with the velocity u = 3.0 and 1st and 2nd order upwind scheme Peter V. Nielsen, Aalborg University pvn@civil.auc.dk 30

32 False Diffusion and Order of the Schemes Flow from an opening, which is inclined at ~30 deg. to the mesh. Three-dimensional flow. Profile at the upper surface at a distance of 1 m from the opening. Dispersive error Diffusive error Svidt Peter V. Nielsen, Aalborg University pvn@civil.auc.dk 31

33 History and Numerical Schemes, 3 The sixties The central difference scheme becomes unstable (wiggly) when the Peclet number is large. The cure is to decrease the grid size. The seventies Upwind difference opened the way for infinitely high Reynolds numbers, but false diffusion could in many cases be larger than diffusion of physical kind. The eighties and the nineties Second order schemes decreased the effect of false diffusion. Peter V. Nielsen, Aalborg University pvn@civil.auc.dk 32

34 CFD in Ventilation Design Computational fluid dynamics in a nutshell Symbols and glossary Mathematical background Turbulence models Numerical methods Boundary conditions Quality control CFD combined with other prediction models Application of CFD codes in building design Case studies Benchmark tests Peter V. Nielsen, Aalborg University pvn@civil.auc.dk 33

35 Boundary Conditions Wall boundary Free boundary Plane of symmetry Air supply opening Air exit opening Obstacle boundary Air supply opening Peter V. Nielsen, Aalborg University 34

36 CFD in Ventilation Design Computational fluid dynamics in a nutshell Symbols and glossary Mathematical background Turbulence models Numerical methods Boundary conditions Quality control CFD combined with other prediction models Application of CFD codes in building design Case studies Benchmark tests Peter V. Nielsen, Aalborg University pvn@civil.auc.dk 35

37 Quality Control Quality control consists of these major steps: - recognize possible sources of errors, - check for them in your own simulations, - estimate the accuracy of the simulations, - improve the simulations, if possible. Main items in this chapter are: - Steps in a CFD simulation - Sources of errors and uncertainties - How to ensure high quality predictions (recommendations) - Questions to ask the CFD engineer about the work reported - Additional advice and remarks - A short check list Peter V. Nielsen, Aalborg University pvn@civil.auc.dk 36

38 Quality Control, Sources of Errors and Uncertainties Some examples: 2D treatment instead of 3D Simplification, modelling level Peter V. Nielsen, Aalborg University 37

39 Cell Quality Monitoring velocities Versus number of cells in the prediction Peter V. Nielsen, Aalborg University 38

40 Cell Quality Peter V. Nielsen, Aalborg University 39

41 Turbulence Model 3D wall jet in a room simulated by a k-epsilon Model and a Reynolds Stress Model. Schälin and Nielsen Peter V. Nielsen, Aalborg University pvn@civil.auc.dk 40

42 Peter V. Nielsen, Aalborg University 41

43 CFD in Ventilation Design Computational fluid dynamics in a nutshell Symbols and glossary Mathematical background Turbulence models Numerical methods Boundary conditions Quality control CFD combined with other prediction models Application of CFD codes in building design Case studies Benchmark tests Peter V. Nielsen, Aalborg University pvn@civil.auc.dk 42

44 CFD Combined with other Prediction Models Peter V. Nielsen, Aalborg University 43

45 The Regional Library of Northern Jutland The Regional Library of Northern Jutland is used as a test building where a combination of BEPS and CFD is used for prediction of energy consumption and indoor climate. = Peter V. Nielsen, Aalborg University pvn@civil.auc.dk 44

46 Contaminant Distribution Peter V. Nielsen, Aalborg University 45

47 CFD in Ventilation Design Computational fluid dynamics in a nutshell Symbols and glossary Mathematical background Turbulence models Numerical methods Boundary conditions Quality control CFD combined with other prediction models Application of CFD codes in building design Case studies Benchmark tests Peter V. Nielsen, Aalborg University pvn@civil.auc.dk 46

48 Application of CFD Codes in Building Design The applications of CFD in buildings may be grouped under the following headings: Prediction of air jet diffusion Room air movement analysis Prediction of contaminant dispersal Modelling emission from materials and equipment in buildings Indoor air quality prediction Thermal comfort assessment Mean age of air and ventilation effectiveness predictions Prediction of fire and smoke spread Wind flow around buildings Peter V. Nielsen, Aalborg University 47

49 CFD in Ventilation Design Computational fluid dynamics in a nutshell Symbols and glossary Mathematical background Turbulence models Numerical methods Boundary conditions Quality control CFD combined with other prediction models Application of CFD codes in building design Case studies Benchmark tests Peter V. Nielsen, Aalborg University pvn@civil.auc.dk 48

50 Case studies on Different Air Distribution Systems Five air distribution systems are compared with each other. They are all installed in the same room, and they all handle the same situation and the same load. Mixing ventilation with end wall mounted diffuser. Return opening below the supply. Vertical ventilation with a textile terminal. End wall mounted return opening at floor level. Displacement ventilation. End wall mounted low velocity diffuser. End wall mounted return opening below ceiling. Mixing ventilation generated by a ceiling mounted radial diffuser. End wall mounted return opening below ceiling. Peter V. Nielsen, Aalborg University pvn@civil.auc.dk 49

51 The Test Room The test room is the IEA Annex 20 room with length, width and height equal to 4.2 m, 3.6 m and 2.5 m. The heat load consists of two PCs, two desk lamps and two manikins producing a total heat load of 480 W. One work place is used in some of the experiments (240 W). Peter V. Nielsen, Aalborg University pvn@civil.auc.dk 50

52 CFD Model Radiation is ignored Convection is estimate to be 50 % of total heat flux Wall, ceiling and floor Surface temperature Mannequins, PC s and lamps Fixed heat flux Peter V. Nielsen, Aalborg University pvn@civil.auc.dk 51

53 Case Study, Mixing Ventilation with Wall Mounted ATD Peter V. Nielsen, Aalborg University 52

54 Simulation of the Diffuser Peter V. Nielsen, Aalborg University 53

55 Mixing Ventilation with End Wall Mounted Diffuser Peter V. Nielsen, Aalborg University 54

56 Case Study, Vertical Ventilation Peter V. Nielsen, Aalborg University 55

57 Vertical Ventilation Radiation ignored, plane-symmetrical solution domain used. (Stratified flow = plane-symmetrical flow, Momentum driven flow may give unsymmetrical flow) Observations Measurements Peter V. Nielsen, Aalborg University pvn@civil.auc.dk 56

58 Vertical Ventilation, Diffuser Peter V. Nielsen, Aalborg University 57

59 Vertical Ventilation, Diffuser 1st order steady state equations, k-ε turbulence model, 300,000 cells Peter V. Nielsen, Aalborg University 58

60 Vertical Ventilation BC: Diffuser F Peter V. Nielsen, Aalborg University pvn@civil.auc.dk 59

61 Vertical Ventilation, Quality Control Monitoring points Velocity in y-direction Peter V. Nielsen, Aalborg University 60

62 Vertical Ventilation Predictions in the whole room n = 5 h grid points Peter V. Nielsen, Aalborg University pvn@civil.auc.dk 61

63 Case Study, Displacement Ventilation Peter V. Nielsen, Aalborg University 62

64 Diffuser for Displacement Ventilation θ u A Non-prepenticular flow must be compensated in flow area to ensure fixed flow A/cos θ Peter V. Nielsen, Aalborg University pvn@civil.auc.dk 63

65 Displacement Ventilation 1 st order steady state equations, k-ε turbulence model, 220,000 cells, Radiation ignored, plane-symmetrical solution domain used. (Stratified flow = plane-symmetrical flow, Momentum driven flow may give unsymmetrical flow) Grid convergence 40 Monitorpunkt 1 Monitorpunkt 2 Monitorpunkt 3 Monitorpunkt 4 35 Fejl i forhold til målte værdier [%] Antal netpunkter Peter V. Nielsen, Aalborg University pvn@civil.auc.dk 64

66 Displacement Ventilation Case: c Peter V. Nielsen, Aalborg University pvn@civil.auc.dk 65

67 Displacement Ventilation Case: d Peter V. Nielsen, Aalborg University pvn@civil.auc.dk 66

68 Case Study, Mixing Ventilation with Ceiling Mounted ATD Peter V. Nielsen, Aalborg University 67

69 Mixing Ventilation with Ceiling Diffuser, Diffuser Models 4-way diffuser Fixed-flow diffuser Diffuser with horizontal surface Peter V. Nielsen, Aalborg University 68

70 Diffuser Models Measurements 4-way diffuser Peter V. Nielsen, Aalborg University 69

71 Diffuser Models Fixed-flow diffuser Diffuser with horizontal surface Peter V. Nielsen, Aalborg University 70

72 CFD Simulations Temperatures Peter V. Nielsen, Aalborg University 71

73 CFD Simulations Maximum velocity at 1.80 m (top boundary of the occupied zone) Peter V. Nielsen, Aalborg University pvn@civil.auc.dk 72

74 Steady and/or Transient Flow Measurements in ceiling region n = 6.02 h -1 n = 3.25 h -1 Peter V. Nielsen, Aalborg University pvn@civil.auc.dk 73

75 Simulation of Transient Flow (Time Dependent Equations) y-velocity at 1.80 m y-velocity at different positions n = 3.25 h -1 Peter V. Nielsen, Aalborg University pvn@civil.auc.dk 74

76 Direct Description of a Diffuser, 1 Diffuser Rectangular cells and multigrid structure, case 1 Unstructured grid, case 2 Peter V. Nielsen, Aalborg University pvn@civil.auc.dk 75

77 Direct Description of a Diffuser, 2 Peter V. Nielsen, Aalborg University pvn@civil.auc.dk 76

78 Direct Description of a Diffuser, 3 CFD simulation of the diffuser based on fine unstructured grid. Prediction based on the PV method. Velocity values are given from the diffuser simulation. Kondo et al Peter V. Nielsen, Aalborg University pvn@civil.auc.dk 77

79 Literature P. V. Nielsen, The Selection of Turbulence Models for Prediction of Room Airflow. ASHRAE Transactions. 1998; Vol. 104, Part 1B. - pp P. V. Nielsen, Indoor Climate Modelling, Chapter in: Per Erik Nilsson, Achieving the Desired Indoor Climate Energy Efficiency Aspects of System Design. The Commtech Group, Studentlitteratur, Lund, D. N. Sørensen and P. V. Nielsen, Quality Control of Computational Fluid Dynamics in Indoor Environments. International Journal of Indoor Environment and Health, Vol. 13, no. 1, pp. 2-17, March A. Schälin and P. V. Nielsen, Impact of Turbulence Anisotropy near Walls in Room Air Flow. Indoor Air, International Journal of Indoor Environment and Health, Vol. 14, No. 3, pp , P. V. Nielsen, A. Restivo and J. H. Whitelaw, Buoyancy-Affected Flows in Ventilated Rooms, Numerical Heat Transfer, Vol. 2, B. Bjerg, K. Svidt, G. Zhang, S. Morsing, J.O. Johnsen, Modeling of air inlets in CFD prediction of airflow in ventilated animal houses, Computers and Electronics in Agriculture, 34, , 2002 P. V. Nielsen, The Description of Supply Openings in Numerical Models for Room Air Distribution. ASHRAE Transactions, Vol. 98, Part 1, P. V. Nielsen, The Box Method - A Practical Procedure for Introduction of an Air Terminal Device in CFD Calculation. - Aalborg: AAU, p. (R ISSN: ). P. V. Nielsen, The Prescribed Velocity Method - A Practical Procedure for Introduction of an Air Terminal Device in CFD Calculation. - Aalborg: AAU, p. (R ISSN: ). Bjerg B., Svidt K., Morsing S., Zhang G, Comparion of Methods to Model a Wall Inlet in Numerical Simulation of Airflow in Livestock Rooms, Proceedings of AgEng2000, International Conference on agricultural engineering, Warwick, UK, L. Davidson, P. V. Nielsen and A. Sveningsson, Modifications of the Model for Computing the Flow in a 3D Wall Jet. Submitted to THMT-03, International Symposium on Turbulence, Heat and Mass Transfer, October 12 17, 2003, Antalya, Turkey. L. Davidson, P. V. Nielsen and C. Topp, Low-Reynolds Number Effects in Ventilated Rooms: A Numerical Study. In: Proceedings of ROOMVENT 2000, Reading, Peter V. Nielsen, Aalborg University pvn@civil.auc.dk 78