Aerodynamic Basics - Revision history

RealHumanBean: /* Boundary Layers */

2021-08-18T00:31:12Z

Boundary Layers

U2berggeist: /* Low-Re effects */

2021-08-17T23:03:53Z

Low-Re effects

U2berggeist: /* Boundary Layers */

2021-08-17T23:02:15Z

Boundary Layers

U2berggeist: /* Boundary Layers */ Quick expansion

2021-08-17T22:41:40Z

Boundary Layers: Quick expansion

U2berggeist at 21:45, 18 August 2020

2020-08-18T21:45:16Z

U2berggeist: Move paper citation to reference

2020-08-18T21:33:03Z

Move paper citation to reference

U2berggeist: Add notes for Boundary Layer section

2020-08-18T21:23:59Z

Add notes for Boundary Layer section

U2berggeist at 21:08, 18 August 2020

2020-08-18T21:08:49Z

U2berggeist: Update math formatting

2020-08-18T21:07:46Z

Update math formatting

Get KAS at 14:12, 23 May 2020

2020-05-23T14:12:46Z

@@ Line 10: / Line 10: @@
 Lift to Drag ratio, <math>C_l/C_d</math>
-== Boundary Layers ==
+==Boundary Layers==
 There are many resources available about what exactly boundary layers are, why they exist, and other useful information. This will focus on FSAE applications, as they are fairly unique.
@@ Line 17: / Line 16: @@
 [https://doi.org/10.2514/3.59830 10.2514/3.59830]</ref>, as it contains many of the fundamentals for the limitations of downforce generation. Most of what follows is a summary of the paper.
-=== Laminar, Turbulent, and Transition ===
+===Laminar, Turbulent, and Transition===
 * Boundary layers (BL) can be either laminar, or turbulent
@@ Line 26: / Line 25: @@
 ** The artificial methods force the transition to occur sooner than it would otherwise
-* Determining when a boundary layer transitions is a function of it's Reynolds Number, <math>Re_x</math>, using <math>x</math> (distance along a wall) as its characteristic length scale
+* Determining when a boundary layer transitions is a function of it's Reynolds Number, <math>Re_x</math>, using <math>x</math> (distance along a wall) as its characteristic length scale. The boundary layer thickness can also be described non-dimensionally as <math>Re_\theta</math> where <math>\theta</math> is the local boundary layer thickness; this defines the boundary layer properties strictly in terms of local variables, as the meaning of "x" becomes less useful when the streamwise pressure gradient varies over the surface of the airfoil.<br />
 ** The higher the Reynolds number the more likely it is to be turbulent (over a certain level, it is more-or-less guaranteed)
 ** Therefore, increasing flow velocity decreases the distance at which BL transition occurs
-=== Pressure Gradients and Separation ===
+===Pressure Gradients and Separation===
 * Flow separation occurs as a direct result of the existence of boundary layers
 ** Hence why "flow separation" and "boundary layer separation" are synonymous
@@ Line 37: / Line 37: @@
 ** A pressure gradient that pushes ''against'' the (bulk) fluid motion is known as an '''Adverse Pressure Gradient (APG)'''
 * APGs are what cause flow separation
-** The APG pushes against the boundary layer such that the flow close to the wall (which is significantly slower than the flow outside the BL) begins to move backwards
+** The APG pushes against the boundary layer such that the flow close to the wall (which is significantly slower than the flow outside the BL) begins to move backwards. At incipient separation, the shear stress at the wall reaches 0. <br />
 * Laminar and Turbulent BL react to APGs differently:
 ** '''''Laminar BLs are more susceptible to separation than turbulent BLs'''''
@@ Line 58: / Line 59: @@
-* Unless you're airfoil is operating outside the Low-<math>Re</math> regime (generally around <math>Re_c \approx 25,000</math>, though this is airfoil dependent), use airfoils explicitly designed for Low-<math>Re</math> regime
+* Unless your airfoil is operating outside the Low-<math>Re</math> regime (generally around <math>Re_c \approx 25,000</math>, though this is airfoil dependent), use airfoils explicitly designed for Low-<math>Re</math> regime
 ** These are meant to place the boundary layer transition point ahead of the APG region of the airfoil (or to "help" the BL transition sooner)
 ** S1223 airfoil is the quintessential airfoil design for this regime (at least for high lift applications, as would be found in FSAE cars)
@@ Line 64: / Line 65: @@
 '''Important Note for CFD:'''
-* Low-<math>Re</math> effects are generally ''NOT'' captured by CFD, as all common turbulence models treat the BL as always turbulent.
+* Low-<math>Re</math> effects are generally ''NOT'' well-captured by CFD, as many common turbulence models treat the BL as always turbulent. Modern commercial CFD codes do include transitional RANS models which are of some utility.
-** There are special "transition" models, but they require empirical calibration to work correctly
+** There are special "transition" models, but they require empirical calibration to work correctly. <br />
-** Even if you have the empirical calibration data, BL transitions is ''very'' sensitive to surface roughness, surface shape, atmospheric conditions, etc.
+** Even if you have the empirical calibration data, BL transitions is ''very'' sensitive to surface roughness, surface shape, atmospheric conditions, etc. FSAE manufacturing processes can be rather crude and a defect near the leading edge has a massive effect on the flow over the rest of the airfoil chord. <br />
-** My advice is to simply trust wind tunnel data (such as [https://m-selig.ae.illinois.edu/uiuc_lsat.html UIUC LSAT]) to determine whether airfoils operating in Low-<math>Re</math>. If they are not, then the CFD assumptions aren't broken. If they are, then don't rely on CFD.
+** My advice is to simply trust wind tunnel data (such as [https://m-selig.ae.illinois.edu/uiuc_lsat.html UIUC LSAT]) to determine whether airfoils operating in Low-<math>Re</math>. If they are not, then the CFD assumptions aren't broken. If they are, then don't rely on CFD. CFD can arguably provide very good results pointing you towards a relatively optimal design even in the transitional regime, but not necessarily for providing a precise number for lift and drag coefficients. Like any other engineering tool, it has its purpose, but must be supplemented by sound engineering reasoning and experimental testing and validation.<br />
 ==Downforce==
 [[Category:Body and Aero]]

@@ Line 52: / Line 52: @@
 * Low-<math>Re</math> phenomenon include:
-** "Premature" separation (compared to running the airfoil at higher <math>Re</math>
+** "Premature" separation (compared to running the airfoil at higher <math>Re</math>)
 ** Hysteresis
 *** The AoA at which separation occurs is different when increasing AoA than decreasing AoA

@@ Line 39: / Line 39: @@
 ** The APG pushes against the boundary layer such that the flow close to the wall (which is significantly slower than the flow outside the BL) begins to move backwards
-* laminar vs turbulent in adverse pressure gradients
+* Low-<math>Re</math> phenomenon include:
-* Subsequent low-Re number effects (really important for airfoil selection and significant implications on CFD)
+** "Premature" separation (compared to running the airfoil at higher <math>Re</math>
 ==Downforce==
 [[Category:Body and Aero]]

@@ Line 12: / Line 12: @@
 == Boundary Layers ==
-Mention/reiterate the contents of ''High Lift Aerodynamics''<ref>A. Smith, “High Lift Aerodynamics,” Journal of Aircraft, vol. 12, no. 6, pp. 501–539, 1975, doi:
+There are many resources available about what exactly boundary layers are, why they exist, and other useful information. This will focus on FSAE applications, as they are fairly unique.
 [https://doi.org/10.2514/3.59830 10.2514/3.59830]</ref>
 * laminar vs turbulent in adverse pressure gradients
 * Subsequent low-Re number effects (really important for airfoil selection and significant implications on CFD)
 ==Downforce==
 [[Category:Body and Aero]]

@@ Line 12: / Line 12: @@
 == Boundary Layers ==
-Mention/reiterate the contents of ''High Lift Aerodynamics''<ref>[1]A. Smith, “High Lift Aerodynamics,” Journal of Aircraft, vol. 12, no. 6, pp. 501–539, 1975, doi:
+Mention/reiterate the contents of ''High Lift Aerodynamics''<ref>A. Smith, “High Lift Aerodynamics,” Journal of Aircraft, vol. 12, no. 6, pp. 501–539, 1975, doi:
 [https://doi.org/10.2514/3.59830 10.2514/3.59830]</ref>

@@ Line 3: / Line 3: @@
 Aerodynamics is not necessary to make a car go around a racetrack. Check beforehand if your car benefits from aerodynamic elements. Simple laptime simulation usually includes lift and drag coefficents and helps you determine your target goals.
 ==Key Parameters==
-Coefficient of Drag,<math>C_d</math>
+Coefficient of Drag, <math>C_d</math>
-Lift to Drag ratio,<math>C_l/C_d</math>
+== Boundary Layers ==
 ==Downforce==
 [[Category:Body and Aero]]

@@ Line 1: / Line 1: @@
-Aerodynamics in FSAE can be used to describe the ineraction of air and the racecar, in particular its bodywork and aerodynamic elements such as wings, diffusers and fins. It can be analysed by the use of [[Computational Fluid Dynamics]]. The performance of aerodynamic elements can be described by parameters such as the coefficient of Lift (<math>C_l</math>), coefficient of Drag (<meta http-equiv="content-type" content="text/html; charset=utf-8"><span><math>C_d</math></span>) and the ratio of <meta http-equiv="content-type" content="text/html; charset=utf-8"><span><math>C_l/C_d</math></span>.
+Aerodynamics in FSAE can be used to describe the interaction of air and the racecar, in particular its bodywork and aerodynamic elements such as wings, diffusers and fins. It can be analysed by the use of [[Computational Fluid Dynamics]]. The performance of aerodynamic elements can be described by parameters such as the coefficient of Lift (<math>C_l</math>), coefficient of Drag (<math>C_d</math></span>) and the ratio of <math>C_l/C_d</math></span>.
 ==Conceptual aspect of Aerodynamics==
-Aerodynamics is not necessary to make a car go around a racetrack. Check beforehand if your car benefits from aerodynamic elements. Simple laptime simulation usually inclues lift and drag coefficents and helps you determine your target goals.
+Aerodynamics is not necessary to make a car go around a racetrack. Check beforehand if your car benefits from aerodynamic elements. Simple laptime simulation usually includes lift and drag coefficents and helps you determine your target goals.
 ==Key Parameters==
-Coefficient of Lift,<meta http-equiv="content-type" content="text/html; charset=utf-8"><span><math>C_l</math></span>
+Coefficient of Lift,<math>C_l</math></span>
-Coefficient of Drag,<meta http-equiv="content-type" content="text/html; charset=utf-8"><math>C_d</math>
+Coefficient of Drag,<math>C_d</math>
-Lift to Drag ratio,<meta http-equiv="content-type" content="text/html; charset=utf-8"><math>C_l/C_d</math>
+Lift to Drag ratio,<math>C_l/C_d</math>
 ==Downforce==
 [[Category:Body and Aero]]