# Laminar Flow. Chapter ZERO PRESSURE GRADIENT

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1 Chapter 2 Laminar Flow 2.1 ZERO PRESSRE GRADIENT Problem Consider a uniform flow of velocity over a flat plate of length L of a fluid of kinematic viscosity ν. Assume that the fluid is incompressible and that the flow is steady and bi-dimensional. The coordinate system has x aligned with the plate, y perpendicular to the plate and the origin at the leading edge of the plate, as illustrated in figure 2.1. Figure 2.1: niform flow of velocity over a flat plate of length L. a) Determine the pressure gradient d p/dx imposed to the boundary-layer and the external velocity e. b) Determine the resistance force of the plate. 1

2 2 CHAPTER 2. LAMINAR FLOW c) Assuming that the boundary-layer thickness corresponds to =.99 e, determine at which distance from the leading edge x/l does the streamline that is.1l away from the plate at the leading edge enters the boundary-layer. d) Is the velocity component normal to the plate positive, zero or negative? Justify your answer. e) In which conditions are the calculations requested above realist? Problem A third degree polynomial is proposed for a zero pressure gradient boundary-layer velocity profile. e = A 1 η 3 + A 2 η 2 + A 3 η + A 4 with η = y δ. is the velocity component parallel to the wall, e is the external velocity, y is the normal distance to the wall, δ is the boundary-layer thickness and A 1, A 2, A 3 and A 4 are constants. a) Determine the constants A 1, A 2, A 3 and A 4. b) Determine the displacement thickness, δ /δ, the momentum thickness, θ/δ, and the shape factor H. c) sing the integral von Kármán equation, determine the boundary layer thickness δ/x as a function of the distance to the leading edge. d) Calculate the evolution of δ /x, θ/x and the skin friction coefficient, C f = τ w / 1 2 ρ 2 e, along the plate. (τ w is the shear-stress at the wall and ρ is the fluid density). e) Determine o the velocity component normal to the plate V. f) Determine the resistance coefficient C D of a plate of length L. Problem Consider a parabolic velocity profile for a boundary-layer over a flat plate given by: e = A 1 + A 2 η + A 3 η 2 with η = y δ. is the velocity component parallel to the plate, e is the external velocity, y is the normal distance to the wall, δ is the boundary-layer thickness and A 1, A 2 and A 3 are constants. a) Determine the constants A 1, A 2 and A 3.

3 2.1. ZERO PRESSRE GRADIENT 3 b) Calculate the dimensionless displacement thickness, δ /δ, momentum thickness, θ/δ, and shape factor H. c) sing the integral von Kármán equation, determine the boundary layer thickness δ/x as a function of the distance to the leading edge. d) Calculate the evolution of δ /x, θ/x and the skin friction coefficient, C f = τ w / 1 2 ρ 2 e, along the plate. (τ w is the shear-stress at the wall and ρ is the fluid density). e) Determine o the velocity component normal to the plate V. f) Determine the resistance coefficient C D of a plate of length L. Problem Consider a velocity profile for a boundary-layer over a flat plate given by: ( π ) ( π ) = A 1 sin e 2 η + A 2 cos 2 η with η = y δ. is the velocity component parallel to the plate, e is the external velocity, y is the normal distance to the wall, δ is the boundary-layer thickness and A 1 and A 2 are constants. a) Determine the constants A 1 and A 2. b) Calculate the dimensionless displacement thickness, δ /δ, momentum thickness, θ/δ, and shape factor H. c) sing the integral von Kármán equation, determine the boundary layer thickness δ/x as a function of the distance to the leading edge. d) Calculate the evolution of δ /x, θ/x and the skin friction coefficient, C f = τ w / 1 2 ρ 2 e, along the plate. (τ w is the shear-stress at the wall and ρ is the fluid density). e) Determine o the velocity component normal to the plate V. f) Determine the resistance coefficient C D of a plate of length L. Problem The velocity profile ( is the velocity component parallel to the plate, e is the external velocity, y is the normal distance to the wall and δ is the boundary-layer thickness) e = 1 e 4.65η with η = y δ

4 4 CHAPTER 2. LAMINAR FLOW is a smooth curve that respects = at y = and =.99 e at y = δ, which could suggest a reasonable approximation for zero pressure gradient boundary-layer. However, the integration of the von Kármán equation with this profile leads to δ x 9.2(Re x).5 with Re x = ex ν, which is almost 1% different from the value of the Blasius solution (with =.99 e for y = δ) δ x 5(Re x).5. What is the reason for the poor accuracy of this result? Figure 2.2: Aerodynamic tunnel with adjustable top wall of length L, height h and inlet velocity o. Problem An aerodynamic tunnel of length L has a top wall with adjustable height h = f (x), as illustrated in figure 2.2. At the inlet of the tunnel, the height is h o and the flow is uniform with a velocity equal to o. Assume that the tunnel as smooth walls, the fluid is incompressible and the flow is steady and two-dimensional. (ν ar = m 2 /s, ρ ar = 1.2 kg/m 3 ). a) Without neglecting the effect of the boundary-layers on the walls, determine the shape of the top wall of the tunnel h = f (x) that leads to zero pressure gradient imposed to the bottom wall boundary-layer. b) Determine the resistance force of the bottom wall of the tunnel. c) For h o =.3m and L = 1, estimate the largest value of o for which the flow inside the tunnel is laminar.

5 2.2. FAVORABLE AND ADVERSE PRESSRE GRADIENT FAVORABLE AND ADVERSE PRESSRE GRADIENT Problem Approximate solutions of boundary-layer flows may be obtained solving the von Kármán integral equation supplemented by the definition of the velocity profile. For the velocity profile given by ( π ) ( = A + Bsen e 2 η +Csen 2 π ) 2 η with η = y δ, where is the velocity component parallel to the plate, e is the external velocity, y is the normal distance to the wall, δ is the boundary-layer thickness and A, B and C are constants: a) Indicate the most appropriate values for A, B and C. b) Determine the parameter λ = θ 2 ν d e dx corresponding to flow separation, (τ w = ), of this type of profiles. c) Determine the constants B and C for the velocity profile of a boundary-layer from a stagnation point flow that exhibits e = ax. d) For a zero pressure gradient boundary-layer, for which regime of flow, laminar or turbulent, is this profile suitable? Justify your answer. sen 2 ( π 2 η ) dη = 1 2, sen 3 ( π 2 η ) dη = 4 3π sen 4 ( π 2 η )dη = 3 8 Problem A boundary-layer velocity profiles is defined by: e = Aln(η + 1) + Bη +C with η = y δ. is the velocity component parallel to the plate, e is the external velocity, y is the normal distance to the wall, δ is the boundary-layer thickness and A, B and C are constants.

6 6 CHAPTER 2. LAMINAR FLOW a) Determine the constants A, B and C. b) Discuss for which boundary-layer flow is this profile suitable. ln(η + 1)dη = 2ln(2) 1, (ln(η + 1)) 2 dη = 2(1 ln(2)) 2 ηln(η + 1)dη =,25 Problem Consider a small wind tunnel with a flat bottom and an adjustable top wall, as illustrated in figure 2.3. Assume that the tunnel is sufficiently wide to assume two-dimensional flow. At the inlet, the velocity profile is uniform and the velocity is equal to o = 25m/s. The length of the tunnel is L = 1m and the height at the inlet is equal to h o =.3m. Figure 2.3: Wind tunnel with adjustable flat top wall of angle α and length L = 1m, inlet height h o =.3m and inlet velocity o = 25m/s. a) Assuming that the effect of the boundary-layers on the tunnel walls is negligible, determine the distribution of the mean velocity along the tunnel as a function of the distance to the inlet x and the angle α. b) sing Thwaites s method, determine the angle α that leads to flow separation at x = L on the bottom wall of the tunnel. c) For the conditions of question b), estimate the value of the momentum thickness, θ, on the bottom wall of the tunnel at x = L. d) For the conditions of question b) and assuming a velocity profile given by e = C 1 +C 2 η +C 3 η 2 +C 4 η 3 with η = y δ, estimate the boundary-layer thickness, δ, on the bottom wall of the tunnel at x = L.

7 2.2. FAVORABLE AND ADVERSE PRESSRE GRADIENT 7 e) Is the situation defined in question b), laminar flow separation at x = L on the bottom wall of the tunnel, physically realistic? Problem Consider a laminar, two-dimensional boundary-layer of an incompressible fluid developing in a region where the external velocity, e, is given by e (x) = o (1 + x/l) 2. Momentum thickness θ is zero for x =. (ν = m 2 /s) a) Determine the pressure gradient dp/dx imposed to the boundary-layer. b) sing Thwaites s method, determine the distance x/l for which the boundary-layer separates. c) Estimate the relation that o and L must satisfy to guarantee that the result of question b) is realist. Problem A wind tunnel of length L = 1m has an adjustable top wall. At the inlet, the tunnel height is h o =.3m and the flow is uniform with a velocity equal to o = 1m/s. Assume that the tunnel has smooth walls, the flow is two-dimensional and the fluid is incompressible. (ν ar = m 2 /s, ρ ar = 1.2 kg/m 3 ) a) Neglecting the effect of the boundary-layers on the tunnel walls, determine the shape of the top wall of the tunnel, h = f (x), that leads to a pressure gradient imposed to the boundary-layer of the bottom wall given by dp dx = ρ o 2 ( 1 + x ). L L b) Determine the evolution of the displacement thickness, θ, with the distance to the inlet, x. c) Determine the distributions of displacement thickness δ and skin friction coefficient, C f = τ w / 1 2 ρ 2 e, along the bottom wall. (τ w is the shear-stress at the wall and ρ is the density of the fluid).

8 8 CHAPTER 2. LAMINAR FLOW Problem Consider a two-dimensional boundary-layer of an incompressible fluid developing in the x direction in the laminar regime. The external velocity is defined by: e = ax, where a is a constant. Assume that the boundary-layer approximations are valid for x. (ν = m 2 /s). a) Determine the pressure gradient imposed to the boundary-layer, dp dx. b) Determine the evolution of the momentum thickness, θ, with the distance to the inlet of the tunnel, x. c) Determine the evolution of the shape factor, H, and of the momentum thickness, δ, with x. Comment the result. d) Determine the evolution of the skin friction coefficient, C f, with x. e) Discuss if there is any value of a for which the solution above looses physical meaning for x < 1m.

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