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Potential Flow 3 CHAPTER Potential Flow 3 LEARNINGOBJECTIVES • Learn to calculate the air flow and pressure distribution around various body shapes. • Learn more about the classical assumption of irrotational flow and its meaning that the vorticity is everywhere zero. Note that this also implies inviscid flow. Irrotational flows are potential fields. • Learn about the potential function, known as the velocity potential. Learn that the velocity components can be determined from the velocity potential. • Learn that the equations of motion for irrotational flow reduce to a single partial differential equation for velocity potential known as the Laplace equation. • Examine the classical analytical techniques described for obtaining two-dimensional and axisymmetric solutions to the Laplace equation for aerodynamic applications. • Learn how computational tools are applied for predicting the potential flows around arbitrary two-dimensional geometries. 3.1 TWO-DIMENSIONAL FLOWS The problem of interest in this section is illustrated in Fig. 3.1, which illustrates the two-dimensional steady motion of a body moving at constant speed U in a quiescent fluid. The quiescent fluid is incompressible with uniform density ρ and uniform pres- sure p . The figure shows this from the point of view of a coordinate system attached to the1 body. Hence, the body appears stationary in a uniform onset flow. In this coor- dinate system, we can assume steady-state conditions. Note that the x direction of the coordinate system illustrated in the figure was selected, for convenience, to be in the direction of the far-field velocity vector; the far-field velocity vector is U Ui. The aerodynamic problem is to determine the velocity, u .u,v/, and1 D pressure p for the flow illustrated in Fig. 3.1 as functions of the coordinatesD .x,y/. Since the flow of interest is a two-dimensional incompressible flow, the continuity equation is Aerodynamics for Engineering Students 149 c 2013 Elsevier Ltd. All rights reserved. 150 CHAPTER 3 Potential Flow y U x FIGURE 3.1 Arbitrary body in a uniform stream. (as discussed in Section 2.4.2) @u @v 0 (3.1) @x C @y D We next assume that the fluid is inviscid (Euler’s “perfect fluid”). We also assume steady flow and neglect body forces (e.g., gravity). In this case, the Euler equations (described in Section 2.6.1) reduce to the following form: @u @u 1 @p u v (3.2) @x C @y D −ρ @x @v @v 1 @p u v (3.3) @x C @y D −ρ @y Equations (3.1), (3.2), and (3.3) are in three unknowns. The unknowns are u, v, and p. Thus, they represent a complete system of equations that describes the flows of interest. We can simplify this system of equations as follows. Equation (3.2) can be rearranged to read @u2=2 @u @p/ρ v @x C @y D − @x Adding and subtracting a term (the second and third term in the following), we get @ u2 v2 @ v2 @u @ p v @x 2 C 2 − @x 2 C @y D −@x ρ 3.1 Two-Dimensional Flows 151 Or, after rearranging terms, @ u2 v2 p @v @u v (3.4) @x 2 C 2 C ρ D @x − @y Similarly, we can operate on and rearrange Eq. (3.3) to get @ u2 v2 p @v @u u (3.5) @y 2 C 2 C ρ D − @x − @y The equation for the z component of the vorticity (the only finite component of the vorticity vector in two-dimensional flow) is (as described in Section 2.7.4) @v @u ζ @x − @y D For irrotational flows, the condition for irrotationality is ζ 0; hence D @v @u 0 (3.6) @x − @y D Applying Eq. (3.6) to Eqs. (3.4) and (3.5) and integrating the latter pair, we get u2 v2 p H (3.7) 2 C 2 C ρ D where H is a constant. This is Bernoulli’s equation. If we evaluate H far upstream where the flow is uniform at speed U and at pressure p , we get 1 u2 v2 p U2 p 1 2 C 2 C ρ D 2 C ρ Rearranging this equation, p p u2 v2 Cp − 1 1 C (3.8) D 1 2 D − U2 2 ρU where Cp is a dimensionless pressure coefficient. Equation (3.6), the irrotationality condition, allows us to define a scalar function φ as follows: @φ @φ u and v (3.9) D @x D @y 152 CHAPTER 3 Potential Flow Substituting this into Eq. (3.6), we get @2φ @2φ @2φ @2φ 0 @x@y − @y@x D @x@y − @x@y D which illustrates the fact that we can write the velocity vector u .u,v/ as the gra- dient of a scalar function φ. That is, if u φ, then u 0 (inD two dimensions, these are Eqs. (3.9) and (3.6), respectively).D r r × D Substituting Eq. (3.9) into the continuity equation, Eq. (3.1), we get @2φ @2φ 0 (3.10) @x2 C @y2 D This is the Laplace equation for φ; hence φ is a harmonic potential function that we call the velocity potential. (Further discussion of this function and its relationship to the streamlines is given in Section 3.1.1.) Equation (3.1), the continuity equation, also allows us to define the stream function as follows: @ @ u , v (3.11) D @y D − @x Substituting this into Eq. (3.6), the irrotationality condition, we get @2 @2 0 (3.12) @x2 C @y2 D This indicates that the stream function (already described in Section 2.5) is also a harmonic potential. In fact, since @φ @ @φ @ and @x D @y @y D − @x lines of the constant value of φ are perpendicular (or orthogonal) to lines of the con- stant value of . The former are equipotential lines, and the latter are streamlines. These relationships are the Cauchy-Riemann equations, and they play an important role in the mathematical justification for the practical procedures we apply herein to solve potential-flow problems. Equation (3.10) or (3.12) indicates that any solution to the Laplace equation is a possible potential flow. Since the Laplace equation is a linear partial differential equation, if, for example, two solutions are summed (linearly superimposed), a third solution is obtained. In other words, we can construct solutions by the superposition of elementary solutions to solve particular potential-flow problems. Before we exam- ine elementary solutions to the Laplace equation, let us summarize the procedure to solve potential-flow problems in general. 3.1 Two-Dimensional Flows 153 Note that any streamline can be a boundary of a body on which we wish to predict the pressure distribution. Streamlines are lines tangent to the velocity vectors; there- fore, in the direction normal (or perpendicular) to a streamline, the normal component of the velocity is zero: @φ n φ 0 on SB (3.13) · r D @n D where SB is the streamline that represents the body surface or the wall of interest in an investigation, and n is the unit vector perpendicular to the streamline. An example of SB is the boundary surrounding the cross-hatched region (or body) in Fig. 3.1. To solve boundary-value problems associated with solving Eq. (3.10), the Laplace equation for φ, we need boundary conditions on all components of the boundary that completely surrounds the flow field of interest. In addition to a condition on SB (i.e., Eq. 3.13), we need to specify the far-field condition, which for the problem illustrated in Fig. 3.1, is φ Ux (3.14) 1 D which is the potential for a uniform stream in the x direction. This completes the specification of the boundary-value problem illustrated in Fig. 3.1. How we con- struct solutions to this class of problems is discussed in this chapter and in the two subsequent chapters. The steps to solve aerodynamics problems can be summarized as follows. The first step is to construct the flow field by either solving for φ or for . Once one of these fields (or functions) is known, then the velocity field is computed by solving Eq. (3.9) or (3.11) given φ or , respectively. Knowing the velocity field, we compute the pressure field by applying Eq. (3.8). Finally, we integrate the pressure distribution on SB appropriately to determine the force and moment acting on SB due to pressure. Before we begin to deal with solving particular aerodynamic problems in Section 3.2, in the next three subsections we examine in more detail the velocity potential. 3.1.1 The Velocity Potential The stream function (see Section 2.5) at a point has been defined as the quantity of fluid moving across some convenient imaginary line in the flow pattern, and lines of constant stream function (amount of flow or flux) may be plotted to give a picture of the flow pattern (see Section 2.5). Another mathematical definition, giving a dif- ferent pattern of curves, can be obtained for the same flow system. In this case, an expression giving the amount of flow along the convenient imaginary line is found. In a general two-dimensional fluid flow, consider any (imaginary) line OP joining the origin of a pair of axes to the point P(x, y). Again, the axes and this line do not impede the flow and are used only to form a reference datum. At a point Q on the line, let the local velocity q meet the line OP in β (Fig.
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