AerE 344 Lecture Notes Lecture # 07: Laminar and Turbulent Flows Dr. Hui Hu Department of Aerospace Engineering Iowa State University Ames, Iowa 50011, U.S.A Copyright © by Dr. Hui Hu @ Iowa State University. All Rights Reserved! Reynolds’ experiment DU Re Copyright © by Dr. Hui Hu @ Iowa State University. All Rights Reserved! Laminar Flows and Turbulence Flows • Laminar flow, sometimes known as streamline flow, occurs when a fluid flows in parallel layers, with no disruption between the layers. Inflow dynamics laminar flow is a flow regime characterized by high momentum diffusion, low momentum convention, pressure and velocity almost independent from time. It is the opposite of turbulent flow. – In nonscientific terms laminar flow is "smooth," while turbulent flow is "rough." • In fluid dynamics, turbulence or turbulent flow is a fluid regime characterized by chaotic, stochastic property changes. This includes low momentum diffusion, high momentum conversation, and rapid variation of pressure and velocity in space and time. – Flow that is not turbulent is called laminar flow Copyright © by Dr. Hui Hu @ Iowa State University. All Rights Reserved! Turbulent Flows in a Pipe VD Re Copyright © by Dr. Hui Hu @ Iowa State University. All Rights Reserved! Characterization of Turbulent Flows u u u'; v v v' w w w' 1 t0 T 1 t0 T 1 t0 T u u(x, y, z,t)dt; v v(x, y, z,t)dt; w w(x, y, z,t)dt T T T t0 t0 t0 Copyright © by Dr. Hui Hu @ Iowa State University. All Rights Reserved! Turbulence intensities u' 0; v' 0 w' 0 1 to T (u')2 (u')2 dt 0; (v')2 0 (w')2 0 T t0 Copyright © by Dr. Hui Hu @ Iowa State University. All Rights Reserved! Turbulent Shear Stress u Laminar flows: lam y u'v' Turbulent flows: turb u u'v' lam turb y A.laminar flow b.turbulent flow Copyright © by Dr. Hui Hu @ Iowa State University. All Rights Reserved! Laminar Flows and Turbulence Flows D C d 1 U 2 A 2 DU Re Copyright © by Dr. Hui Hu @ Iowa State University. All Rights Reserved! Flow Around A Sphere with laminar and Turbulence Boundary Layer Copyright © by Dr. Hui Hu @ Iowa State University. All Rights Reserved! Aerodynamics of Golf Ball Copyright © by Dr. Hui Hu @ Iowa State University. All Rights Reserved! Laminar Flows and Turbulence Flows 1.0 ) 0.8 0.6 0.4 0.2 smooth-ball 0 rough-ball Centerline Velocity (U/U golf-ball Re=100,000 -0.2 -0.4 -2.0 -1.5 -1.0 -0.5 0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 Distance (X/D) Smooth ball Rough ball Golf ball U m/s: -10 -5 0 5 10 15 20 25 30 35 40 2 U m/s: -10 -5 0 5 10 15 20 25 30 35 40 2 U m/s: -10 -5 0 5 10 15 20 25 30 35 40 2 1 1 1 /D /D /D Y Y Y 0 0 0 -1 -1 -1 4 3 2 1 0 -1 4 3 2 1 0 -1 4 3 2 1 0 -1 X/D X/D X/D Copyright © by Dr. Hui Hu @ Iowa State University. All Rights Reserved! Automobile aerodynamics Copyright © by Dr. Hui Hu @ Iowa State University. All Rights Reserved! Automobile Aerodynamics Mercedes Boxfish Vortex generator above a Mitsubishi rear window Copyright © by Dr. Hui Hu @ Iowa State University. All Rights Reserved! CONVENTIONAL AIRFOILS and LAMINAR FLOW AIRFOILS • Laminar flow airfoils are usually thinner than the conventional airfoil. • The leading edge is more pointed and its upper and lower surfaces are nearly symmetrical. • The major and most important difference between the two types of airfoil is this, the thickest part of a laminar wing occurs at 50% chord while in the conventional design the thickest part is at 25% chord. • Drag is considerably reduced since the laminar airfoil takes less energy to slide through the air. • Extensive laminar flow is usually only experienced over a very small range of angles-of-attack, on the order of 4 to 6 degrees. • Once you break out of that optimal angle range, the drag increases by as much as 40% depending on the airfoil Copyright © by Dr. Hui Hu @ Iowa State University. All Rights Reserved! Flow Separation on an Airfoil Copyright © by Dr. Hui Hu @ Iowa State University. All Rights Reserved! Aerodynamic Performance of An Airfoil 60 1.4 L C 40 1.2 l 1 V 2c 2 20 l 1.0 Airfoil stall 0 0.8 C*100 Before stall / GA(W)-1 airfoil Y -20 Lift Coefficient, C Coefficient, Lift 0.6 25 m/s C =2 shadow region L Experimental data -40 0.4 -60 vort: -4.5 -3.5 -2.5 -1.5 -0.5 0.5 1.5 2.5 3.5 4.5 0.2 0 2 4 6 8 10 12 14 16 18 20 -40-200 20406080100120140 X/C*100 Angle of Attack (degrees) 60 0.40 0.35 40 0.30 d 20 0.25 Experimental data 0 0.20 C*100 / Y D GA(W)-1 airfoil After stall 0.15 Cd Drag Coefficient, C Coefficient, Drag 1 2 -20 V c 25 m/s 0.10 2 shadow region -40 0.05 Airfoil stall vort: -4.5 -3.5 -2.5 -1.5 -0.5 0.5 1.5 2.5 3.5 4.5 0 0 2 4 6 8 10 12 14 16 18 20 -20 0 20 40 60 80 100 120 X/C*100 Angle of Attack (degrees) Copyright © by Dr. Hui Hu @ Iowa State University. All Rights Reserved! Flow Separation and Transition on Low-Reynolds-number Airfoils • Low-Reynolds-number airfoil (with Re<500,000) aerodynamics is important for both military and civilian applications, such as propellers, sailplanes, ultra-light man- carrying/man-powered aircraft, high-altitude vehicles, wind turbines, unmanned aerial vehicles (UAVs) and Micro-Air-Vehicles (MAVs). • Since laminar boundary layers are unable to withstand any significant adverse pressure gradient, laminar flow separation is usually found on low-Reynolds-number airfoils. Post- separation behavior of the laminar boundary layers would affect the aerodynamic performances of the low-Reynolds-number airfoils significantly • Separation bubbles are usually found to form on the upper surfaces of low-Reynolds-number airfoils . Separation bubble would burst suddenly to cause airfoil stall at high AOA when the adverse pressure gradient becoming too big. Copyright © by Dr. Hui Hu @ Iowa State University. All Rights Reserved! Surface Pressure Coefficient distributions (Re=68,000) Separation point Turbulence transition -3.0 AOA = 06 deg AOA = 08 deg Reattachment point -2.5 AOA = 09 deg AOA = 10 deg -2.0 AOA = 11 deg AOA = 12 deg -1.5 AOA = 14 deg P -1.0 C -0.5 0 0.5 1.0 Typical surface pressure distribution when a laminar 1.5 separation bubble is formed (Russell, 1979) 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 X / C 0.15 12.0 0.10 Transition 0.05 11.5 Reattachm ent Separation 11.0 Y/C 0 -0.05 10.5 -0.10 10.0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 9.5 AOA (degree) AOA X/C 9.0 GA (W)-1 airfoil 8.5 (also labeled as NASA LS(1)-0417 ) 8.0 7.5 Copyright © by Dr. Hui Hu @ Iowa0 State0.05 University.0.10 0.15 All Rights0.20 0.25Reserved!0.30 0.35 0.40 0.45 0.50 X/C Laminar Separation Bubble on a Low-Reynolds-number Airfoil 10 10 5 C*100 5 / C*100 Y 10 m/s / GA (W)-1 airfoil Y separation reattachment 0 Spawise GA (W)-1 airfoil vorticity 0 -18.0 -14.0 -10.0 -6.0 -2.0 2.0 6.0 10.0 14.0 18.0 U m/s: 2.0 4.0 6.0 8.0 10.0 12.0 14.0 16.0 18.0 -5 0 5 10 15 20 25 30 35 X/C*100 -5 0 5 10 15 20 25 30 35 X/C*100 Instantaneous flow field Ensemble-averaged flow field 11 11 10 10 9 9 8 8 7 7 Y/C*100 6 Y/C*100 6 5 5 Reattachment 10 m/s Spanwise 10 m/s 4 Vorticity 4 3 (1/s * 10 ) -25.0 -20.0 -15.0 -10.0 -5.0 0.0 5.0 10.0 15.0 20.0 25.0 Um/s: 2.00 4.00 6.00 8.00 10.00 12.00 14.00 16.00 18.00 3 3 18 20 22 24 26 28 30 32 34 18 20 22 24 26 28 30 32 34 X/C*100 X/C*100 PIV measurement results at AOA = 10 deg, Re=68,000 (Hu et al., ASME Journal of Fluid Engineering, 2008) Copyright © by Dr. Hui Hu @ Iowa State University. All Rights Reserved! Stall Hysteresis Phenomena • Stall hysteresis, a phenomenon where stall inception and stall recovery do not occur at the same angle of attack, has been found to be relatively common in low-Reynolds-number airfoils. • When stall hysteresis occurs, the coefficients of lift, drag, and moment of the airfoil are found to be multiple- valued rather than single-valued functions of the angle of attack. • Stall hysteresis is of practical importance because it produces widely different values of lift coefficient and lift-to-drag ratio for a given airfoil at a given angle of attack.