Wind Tunnel Testing: Setup and Procedures
MAE 5930, Rocket Systems Design Wind Tunnel Types Closed Circuit Tunnel (P0 )tunnel ! pambient
MAE 5930, Rocket Systems Design 2 Wind Tunnel Types (2)
Our Tunnel is an “Open Circuit” Tunnel
(P0 )tunnel ! pambient
MAE 5930, Rocket Systems Design 3 About Wind Tunnel Testing Key issues • Maintaining Reynolds number \, Mach number similarity • Accounting for possible flow non-uniformities in the wind tunnel • Wind Tunnel Blockage • Effects of the model support system. • Leaks and Losses • Open or Closed Circuit Tunnel
Turbulent or Laminar Flow? • In most cases the full scale flight conditions will produce turbulent flow. • In wind tunnels, low Reynolds number will result in a significant amount of laminar flow. • To achieve a boundary layer similar flight Reynolds number, the boundary layer is often "tripped" to force transition to turbulent flow.
MAE 5930, Rocket Systems Design 4 Laminar and Turbulent Boundary Layers
• Laminar • Turbulent
MAE 5930, Rocket Systems Design 5 Laminar and Turbulent Boundary Layers (2)
• Location of transition point strongly a function of Reynolds number
Laminar
flow
Transitional
Turbulent
• Laminar --> orderly • Turbulent --> chaotic
MAE 5930, Rocket Systems Design 6 Comparison of Reynolds Number and
Mach Number
Mach number is a measure of the ra o of the fluid Kine c energy to the fluid internal energy (direct mo on To random thermal mo on of gas molecules) -- Fundamental Parameter of Compressible Flow --
$ V 2 ' V $ 2 ' %& 2 () M = = & ) " c % ! "(! # 1)( e
Reynold’s number is a measure of the ra o of the Iner al Forces ac ng on the fluid -- to -- the Viscous Forces Ac ng on the fluid -- Fundamental Parameter of Viscous Flow --
!VL L !V 2 Inertial & Forces Re = = 2 # % µ " $ wall Viscous & Forces
MAE 5930, Rocket Systems Design 7 About Wind Tunnel Testing (2)
V! !" #V" # L flight (Re ) flight = µ" L flight
(Re ) ! (Re )wind flight tunnel Vtest
!test "Vtest " Lmodel (Re )wind = tunnel µtest Lmodel
MAE 5930, Rocket Systems Design 8 About Wind Tunnel Testing (3)
Trip Strip along leading edge of a wind tunnel model airfoil
MAE 5930, Rocket Systems Design 9 Types of Drag
Skin Fric on Strongly Effected by Boundary Layer state
W.H. Mason, Virginia Tech MAE 5930, Rocket Systems Design 10 Empirical Skin Friction Correlations
“Smooth Flat Plate with No Pressure Gradient” “Schoenherr Plot”
Re~10,000,000
Effect of Compressibility
Re~500,000
MAE 5930, Rocket Systems Design 11 Wind Tunnel Blockage Effects
Atest = Ai ! Amodel m AV ! = !i i i
Ai
Ai Vtest ! #Vi Ai " Amodel
MAE 5930, Rocket Systems Design 12 Wind Tunnel Blockage Effects (2) NASA Ames Blockage Criterion for Low Speed, Subsonic Flow
4
Assumes Airspeed correc ons for Flow expansion around model
(CD )measured (CD )corrected = 2 ( " % + * 1 $ Amodel ' - *1+ ! - 4 $ Atest ' Herriot, John G.: Blockage Corrections for Three-Dimensional-Flow Closed-Throat Wind * # sec tion & - Tunnels, with Consideration of the Effect of Compressibility. NACA TR 990, 1950 ) ,
MAE 5930, Rocket Systems Design 13 Wind Tunnel Blockage Effects (3) Example, 10% Model Blockage, Measured Airspeed 50 m/sec
Actual airspeed at Model Maximum cross section … 55 m/sec
Measured Drag Coefficient = 0.4 … Corrected drag Coefficient
(CD )corrected =
Herriot, John G.: Blockage Corrections for Three-Dimensional-Flow Closed-Throat Wind Tunnels, with Consideration of the Effect of Compressibility. NACA TR 990, 1950
MAE 5930, Rocket Systems Design 14 Wind Tunnel Instrumentation
MAE 5930, Rocket Systems Design 15 Wind Tunnel Instrumentation (2)
MAE 5930, Rocket Systems Design 16 Wind Tunnel Instrumentation (3)
MAE 5930, Rocket Systems Design 17 Wind Tunnel Instrumentation (4)
MAE 5930, Rocket Systems Design 18 Wake Survey Results
MAE 5930, Rocket Systems Design 19 Questions??
MAE 5930, Rocket Systems Design 20