Summary of Finite Control Volume Analysis in Fluid Mechanics

Summary of Finite Control Volume Analysis in Fluid Mechanics Ole S. Madsen April 6, 2006 At this point we have developed a set of tools from basic conservation prin­ ciples for use in the analysis of fluid mechanics problems. We have developed (derived) these tools (equations) by applying fundamen­ tal conservation laws (e.g. conservation of mass, volume, linear momentum and energy) to finite control volumes as opposed to elementary representative vol­ umes (ERVs). When properly applied, our finite control volume tools will result in the determination of bulk features of the fluid flow - the discharge, Q, the average velocity, V = Q/A, and the total integrated force due to pressures and shear stresses, Fp and Fτ , etc. These bulk features are often sufficient to solve practical problems in hydraulics. If detailed flow features such as velocity dis­ tributions, v(y), pressure distributions, p(x, y), and shear stress distributions, τ (x, y), are required, then ERV analysis based on differential equations must be employed (i.e. the tools of hydrodynamics). The purpose of this write-up is to present a concise summary of the tools we have at our disposal for the analysis of fluid flow problems, along with some simplifications and guidance that should make it easier to apply these tools in a proper fashion. 1 Picking the Control Volume In choosing the control volume for analysis, the most important decision is to pick inflow and outflow areas, Ain and Aout , where the flow is well behaved. Well behaved flow implies that streamlines are straight and parallel. The flow area, A, is chosen so that it is perpendicular to the streamlines. With this choice, the pressure varies hydrostatically in the direction perpendicular to the streamlines. Thus, the pressure varies linearly over the flow area A, and the pressure force on A is perpendicular to A with a magnitude of � P = p dA = pCG,A · A (1.1) A where pCG = pressure at center of gravity (CG) of A. 1 (1.2) Figure 1: Straight and parallel streamlines passing through the flow area (left); linearly varying pressure (right). Also, if the flow is well behaved, it must be nearly uniform in the vicinity of A such that the detailed velocity variation across A, v, will not be excessive. Thus, we have Q = volume flow rate across A = Discharge � = v dA = V A (1.3) A which defines the cross-sectional average velocity V = Q/A = average velocity. (1.4) One can, with reasonable certainty, expect the local deviation of the velocity, v � = v − V , to be relatively small compared to V over most of A. For example, if |v � /V | = δm ≈ 0.1 over most of A, the momentum coefficient Km 1 =1+ A � � A v� V �2 2 ≈ 1 + 0.01 ≈ 1, dA ≈ 1 + δm and the energy coefficient 2 Ke ≈ 1 + 3δm ≈ 1 + 0.03 ≈ 1 can, without actually knowing v, safely be taken as Km = Ke ≈ 1. (1.5) These are the features that make the choice of inflow and outflow areas where flow is well-behaved a must when picking your finite control volume. The con­ trol volume itself is everything between the inflow and outflow areas occupied by the fluid of interest. 2 Notes of caution: 1. If inflow and/or outflow areas have a free surface, the pressure starts at p = 0 at the free surface, and pCG = ρghCG where hCG is the vertical distance of CG of A below the free surface. Thus, pCG is known if the location of free surface is known. 2. If the flow is in a closed conduit, the pressure at the top of the conduit is p0 and this is not (necessarily) zero. So for a closed conduit (pipe) flow, pCG = p0 + ρghCG , where hCG is the vertical distance from the top of the conduit to CG of A. This is not known unless p0 is known! 3. If the flow area is a freely falling jet such that A has air around it and nothing to support its weight (such as a “bottom”), then the effect of gravity makes the fluid accelerate in the vertical direction, az = −g, and the pressure is patm = 0 throughout the fluid. Thus pCG = 0 as long as A is chosen where flow is well behaved [at vena contracta]. 2 Conservation of Mass and Volume With M˙ = rate of mass flow across A = control volume states: � M˙ in − � � A ρv dA, mass conservation for a finite ∂ [mass of fluid in CV] M˙ out = ∂t = 0 (for steady conditions). (2.1) If ρ is constant and the fluid is incompressible, conservation of volume (a.k.a. continuity) states: � Qin − � ∂ [volume of fluid in CV] ∂t = 0 (for steady conditions). Qout = 3 (2.2) 2.1 Continuity equation for a stream tube/conduit The following applies only for one inflow area and one outflow area, and only under steady conditions: Qin = Qout = Q = Discharge = Constant (2.3) and with Q given by (1.3), Q=VA ⇒ Constant ⇒ V = Q . A (2.4) Thus, along a streamline tube (conduit) of varying cross-sectional area, A, the velocity V varies inversely proportional to A. This allows us to relate velocities at two flow areas Q = V 1 A1 = V 2 A2 ⇒ V1 = A2 V2 A1 or V2 = A1 V1 . A2 (2.5) If Q is known we can use (2.4) to obtain V wherever A is known. If Q is not known, we can use (2.5) to express V1 in terms of V2 (or vice versa) thereby reducing the number of unknowns by one. Note: Continuity, i.e. Q = V A, is virtually always one of the tools that you need to employ when solving a fluid mechanics problem. 3 Conservation of Momentum Conservation of momentum for a finite control volume under steady conditions states that ⎧ ⎫ −−−→ � −−−−−−−−→ � � ⎨� ⎬ � −−→ � − ∂ all other forces, gravity force + = MP + ρ�qd∀ = 0 ⎭ ∂t ⎩ on fluid in CV on fluid in CV CV where � � MP = thrust at flow area = Km ρV 2 + pCG A (3.1) (3.2) is perpendicular to A and points inwards (i.e. towards the CV), regardless of whether the flow area is an inflow or outflow area. Note that Km ≈ 1 (see 1.5). Since the momentum principle (3.1) deals with forces, it is obviously of use when looking for estimates of forces from a fluid on its surroundings � � −−→ −−→ � � all other f−− � � all other f−− orces on fluid orces on fluid =− in CV from surroundings in CV on surroundings (3.3) −−−→ −−−−−−−−−→ or, it may be used if “all other forces” and the gravity force are known (or can be estimated) to obtain a relationship between the MPs, i.e. velocities and pres­ sures, at inflow/outflow areas. 4 Important: (3.1) is a vector equation so it has 3 components (one in each of x, y, and z). MPx = MP cos θ (3.4) MPy = − MP sin θ When can gravity be neglected? 1. When forces in the (x, y)-plane are desired since �g acts in the negative z-direction. 2. When the gravity force = ρg∀0 � momentum force = ρV 2 A, i.e. when ∀0 /2 V2 = a length � = velocity head. A 2g (3.5) When can shear stress (frictional) forces be neglected? 1. When the frictional force is � momentum force, i.e. when τs Aτ � ρV 2 A or with τs = when 1 ρf V 2 8 f 0.02 1 A = flow area . � ≈ = (3.6) 8 8 400 Aτ = wall area where f is the Darcy-Weisbach friction factor ≈ 0.02. This corresponds to a “short” transition. 5 The momentum principle is used: 1. to compute forces when MPs are known, 2. to solve for ps and/or V s when forces are “known” (also need continuity), 3. whenever there is an unknown head loss (expanding flow) and forces may be approximately evaluated (or neglected). 4 Energy Conservation - Bernoulli Principle Ė = rate of flow of mechanical energy across A (4.1) = ρgQH V2 pCG + + zCG 2g ρg where Ke ≈ 1 [see (1.5)] H = Total Head = Ke 2 V2 (Q/A) = = Velocity Head 2g 2g pCG = Pressure Head (pCG = pressure at CG of A) ρg zCG = Elevation Head (zCG = elevation of CG of A) 6 (4.2) ⎫ ⎪ ⎪ ⎪ ⎪ ⎬ ⎪ ⎪ ⎪ ⎪ ⎭ (4.3) 4.1 Conservation of Mechanical Energy � Ėin − � Ėout = E˙ diss + ∂ ∂t � � 1 ρgz + ρ�q 2 d∀ 2 CV � (4.4) = Ėdiss (for steady conditions) ∂ ∂t � � 1 ρgz + ρ�q 2 d∀ = rate of change of mechanical energy within CV 2 CV � Ėdiss = rate of dissipation of mechanical energy within CV (4.5) For a streamtube (conduit) having one inflow area and one outflow area: Ėin − E˙ out = ρgQin Hin − ρgQout Hout = E˙ diss (4.6) but Qin = Qout = Q (2.3), so Hin − Hout = Ėdiss = ΔHCV ρgQ ΔHCV = head loss within CV = 4.2 Ėdiss ρgQ (4.7) (4.8) Generalized Bernoulli Principle Flow is from (1) to (2): H 1 = H2 + � ΔH (4.9) 1→ 2 4.2.1 Frictional Head Loss (in straight sections of length �) ΔHf = f =f � V2 τs P �=f ρgA (4Rh ) 2g � � V (4Rh ) ε Re = , ν (4Rh ) (4.10) (4.11) = Darcy-Weisbach Friction Factor (from Moody Diagram) Flow Area A = Wetted Perimeter P 4Rh = D = pipe diameter if the conduit is a circular pipe Rh = Hydraulic Radius = 7 (4.12) 4.2.2 Minor Head Loss ΔHm = KL V2 2g (4.13) KL = minor loss coefficient (4.14) Minor losses are associated with expansions of flow. ⎫ V2 2 ⎪ ⎪ ΔHexp ⎪ ⎪ 2g ⎪ ⎪ ⎪ 2⎪ ⎪ V1 ⎪ ⎬ If A2 � A1 (outflow to large reservoir): ΔHm = 2g (4.15) ⎪ ⎪ A1 /A2 = CV = contraction coefficient for inflow with sharp edges: ⎪ ⎪ ⎪ � �2 2 ⎪ ⎪ ⎪ 1 V2 ⎪ ⎪ ⎭ ΔHm = −1 CV 2g (V1 − V2 )2 = = 2g � A1 1− A2 �2 V1 2 = 2g � A2 −1 A1 �2 • When is H1 ≈ H2 ? or When is ΔH1→2 ≈ 0? ΔH ≈ 0 for a short transition of a converging flow. Short means that ΔHf = f V2 � V2 � (friction is negligible). 4Rh 2g 2g �P wall friction area 4 � ∼ = (4.16) = 200 A flow area f [Note: ∼ same as for momentum, (3.6)] Converging flow means velocity increases (pressure decreases) in the direction of flow. • When is ΔH1→2 �= 0? Whenever the distance from (1) to (2) is not short (c.f. (4.16)) and fric­ tional losses therefore are non-negligible; or whenever there is a flow ex­ pansion (decrease in velocity in flow direction) between (1) and (2). 8 • When can V2 2g be neglected? � �2 2 A2 A2 V1 2 V2 V2 ⇒ = A1 A1 2g 2g � �2 2 V1 A2 ⇒ � 1: can be neglected. A1 2g V1 = V1 2 V2 2 � 2g 2g (4.17) • When can elevation differences be neglected? �� 2 �� � 2� � V1 V V2 2 �� � |z2 − z1 | = Δz � � ≈ max : Δz can be neglected − 2g 2g � 2g (4.18) 4.3 Energy Grade Line (EGL) zEGL = total head = 4.4 pCG V2 + + zCG 2g ρg (4.19) Hydraulic Grade Line (HGL) zHGL = piezometric head = pCG V2 + zCG = zEGL − ρg 2g (4.20) If there is no pump in the pipe system: 1. zEGL always decreases in the direction of flow (ΔH > 0) or stays constant (in cases when ΔH ≈ 0). 2. zHGL is always a distance of V 2 /2g (one velocity head) below the EGL. zHGL can increase in the flow direction when V 2 /2g decreases due to a flow expansion. (See figure on next page.) 4.5 Application of Bernoulli Principle H1 = = � p1 V1 2 + z 1 = H2 + ΔH + 2g ρg 1→ 2 � � V2 2 p2 + ΔHm + z2 + ΔHf + 2g ρg 1→ 2 1→ 2 9 (4.21) 1. If H1 and H2 are known (e.g. free surface elevations in large reservoirs), the head loss between (1) and (2) is known. Expressing all the head loss 2 terms in terms of V 2 /2g = (Q/A) /2g leads to an equation for V (or Q) that can be solved. Since frictional losses depend on “f ” which in turn depends on Re= V (4Rh )/ν, i.e. on the unknown V = Q/A, an iterative solution is required (e.g. guessing the value of “f ” [0.02 or value for fully rough turbulent flow f (�/(4Rh ))] to get a preliminary value V1 , then improving the guess on “f ”, etc.) 2. If the discharge Q is known, the head loss between (1) and (2) can be evaluated, and the necessary head, e.g. H1 , required to drive the specified flow rate through the system can be obtained. 3. If Q and H1 and H2 are given, the pipe system connecting (1) and (2) can be dimensioned. 5 Conclusions • Always use Conservation of Mass/Conservation of Volume to reduce the number of unknowns, Q = V A ⇒ V1 = (A2 /A1 ) V2 or V2 = (A1 /A2 ) V1 , or to calculate velocities, V = Q/A, if Q is known. • If looking for an answer involving forces, always use Conservation of Mo­ mentum. Also, use this when forces are known (or may be estimated with confidence) to obtain the relationship between pressures and veloc­ ities. Once solved, the Bernoulli Principle can be used to obtain head loss (which must always be ≥ zero). Prime example: Short transition of expanding flow. • If head loss between (1) and (2) is known (or may be estimated with confidence, or expressed in terms of unknowns), use the Bernoulli Principle to get the relationship between the pressures and velocities at (1) and 10 (2). Once solved, the Momentum Principle can be used to evaluate forces on/from surroundings. Prime examples: short transition of converging flows (ΔH ≈ 0); pipe flow analysis (ΔH = ΔHf + ΔHm ). • Success depends crucially on the choice of CV ; in particular, well behaved flow area identification. 11