Upload
phungliem
View
222
Download
1
Embed Size (px)
Citation preview
VVR 120 Fluid VVR 120 Fluid MechanicsMechanics
6. Basic definitions, basic equations I (4.2-4.4)
• Stationary and non-stationary flow, streamline, streamtube
• One-, two-, and three-dimensional flow• Laminar and turbulent flow• Reynolds´ number• System and control volume• Continuity equationExercises: C1, C2, C4, and C7
VVR 120 Fluid VVR 120 Fluid MechanicsMechanics
CLASSIFICATION OF FLOWS
Flow characterized by two parameters – time and distance.
Division of flows with respect to time:• Steady flow – time independent• Unsteady flow – time dependent• Quasi-steady flow – slow changes with time
Division of flows with respect to distance:• Uniform flow – constant section area along flow path• Non-uniform flow – variable section area
VVR 120 Fluid VVR 120 Fluid MechanicsMechanics
Examples of flow types:
Steady uniform flow: flowrate (Q) and section area (A)are constant
Steady non-uniform flow: Q = constant, A = A(x).
Steady = time independentUniform = constant section
VVR 120 Fluid VVR 120 Fluid MechanicsMechanics
Unsteady uniform flow: Q = Q(t), A = constant
Unsteady non-uniform flow: Q = Q(t), A = A(x).
Steady = time independentUniform = constant section
VVR 120 Fluid VVR 120 Fluid MechanicsMechanics
VISUALIZATION OF FLOW PATTERNS
Streamline: a curve that is drawn in such a way that it is tangential to the velocity vector at any point along the curve. A curve that is representing the direction of flow at a given time. No flow across a stream line.
VVR 120 Fluid VVR 120 Fluid MechanicsMechanics
Streamtube: A set of streamlines arranged to form an imaginarytube. Ex: The internal surface of a pipeline
VVR 120 Fluid VVR 120 Fluid MechanicsMechanics
Potential flow: Flow that can be represented by streamlines.
Streakline = path made by injected colour in a flow field
VVR 120 Fluid VVR 120 Fluid MechanicsMechanics
Example streamline and streakline
A flowfield is periodic in such a way that the streamline pattern is repeated at fixed intervals. During the first second the fluid is moving upwards to the right at a 45o angle and during the next second the fluid is moving downwards to the right at a 45o angle etcaccording to Fig. a). The flow velocity is constant = 10 m/s. After 2.5 s the particle track for a particle that is released at point A at time zero is shown in Fig. b). If colour is injected continuously at point A from time 0 how will the resulting streakline look like after 2.5 s?
Fig. a) Streamlines Fig. b) Particle track
A •
VVR 120 Fluid VVR 120 Fluid MechanicsMechanics
TWO WAYS OF DESCRIBING FLUID MOTION
• Lagrangian view: the path, density, velocity and other characteristics of each fluid particle in a flow is traced.
• Eulerian view: study the flow characteristics (velocity, pressure, density, etc.) and their variation with time at fixed points in space.
VVR 120 Fluid VVR 120 Fluid MechanicsMechanics
LAMINAR AND TURBULENT FLOW
Laminar flow • Flow along parallel paths• Shear stress proportional to
velocity gradient (τ = μ⋅du/dy)• Disturbances in the flow are
rapidly damped by viscous action
Turbulent flow• Fluid particles moves in a
random manner and not in layers
• Length scales >> molecular scales in laminar flow
• Rapid continuous mixing• Inertia forces and viscous
forces of importance
VVR 120 Fluid VVR 120 Fluid MechanicsMechanics
Reynolds experiment
• Small velocities ⇒ line of dye intact, movement in parallel layers ⇒ laminar flow
• High velocities ⇒ rapid diffusion of dye, mixing ⇒turbulent flow
• Critical velocity ⇒ line of dye begin to break-up, transition between laminar and turbulent flow
Q and P variables
VVR 120 Fluid VVR 120 Fluid MechanicsMechanics
Reynolds´ number
• Reynolds generalized his results by introduction of a dimensionless number (Reynolds number):
μρ
νVDVDR == ν= μ/ρ, V=Q/A
ν = kinematic viscosityμ = dynamic viscosityD = diameter (for pipes)
e
VVR 120 Fluid VVR 120 Fluid MechanicsMechanics
Reynolds numbers for pipe flow
• Laminar flow Re < 2000• Transitional flow Re = 2000 to 4000• Turbulent flow Re > 4000
Two thresholds:Upper critical velocity – transition of laminar flow to
turbulent flowLower critical velocity – transition of turbulent flow to
laminar flow
VVR 120 Fluid VVR 120 Fluid MechanicsMechanics
The critical Reynolds number, Rc, defining the divisionbetween laminar and turbulent flow, is very dependent onthe geometry for the flow.
• Parallel walls: Rc ≅ 1000 (using mean velocity V and spacing D)
• Wide open channel: Rc ≅ 500 (using mean velocity V and depth D)
• Flow about sphere: Rc ≅ 1 (using approach velocity V and sphere diameter D)
VVR 120 Fluid VVR 120 Fluid MechanicsMechanics
C1 When 0.0019 m3/s of water flow in a 76 mm pipeline at 20°C, is the flow laminar or turbulent?
VVR 120 Fluid VVR 120 Fluid MechanicsMechanics
VVR 120 Fluid VVR 120 Fluid MechanicsMechanics
C2 What is the maximum speed at which a spherical sand grain of diameter 0.254 mm may move through water (20°C) and the flow regime be laminar?
VVR 120 Fluid VVR 120 Fluid MechanicsMechanics
FLUID SYSTEM AND CONTROL VOLUME
• Fluid system: Specified mass of fluid within a closed surface
• Control volume: Fix region in space that can’t be moved or change shape. Its surface is called control surface.
VVR 120 Fluid VVR 120 Fluid MechanicsMechanics
CONTINUITY EQUATION
• Steady flowρ1⋅V1⋅A1 = ρ2⋅V2⋅A2 (m1 = m2)
• Incompressible flowV1⋅A1 = V2⋅A2 or Q1 = Q2 (Q = V⋅A)
V: Average velocity at a section (m/s)A: Cross-section area (m2) Q: Flow rate (m3/s)
m1 m2
Control volume
Fluid system volume
VVR 120 Fluid VVR 120 Fluid MechanicsMechanics
Continuity equation applied to changingpipe diameter
Q = constant, A =(x)V1⋅A1 = V2⋅A2 or Q1 = Q2(Q = V⋅A)
Q1=V1 ⋅ A1
Q1Q2
Q2=V2 ⋅ A2
Control volume
VVR 120 Fluid VVR 120 Fluid MechanicsMechanics
• Flow in a pipe junction
Q1 + Q2 = Q3 or V1⋅A1 + V2⋅A2 = V3⋅A3
• Channel flow (unsteady)
d(Vol)/dt = Q1– Q2
(Q12 = 0)
Vol: Volume of water in channel between section 1 and 2
VVR 120 Fluid VVR 120 Fluid MechanicsMechanics
C4 Water flows in a pipeline composed of 75 mm and 150 mm pipe. Calculate the mean velocity in the 75 mm pipe when that in the 150 mm pipe is 2.5 m/s. What is its ratio to the mean velocity in the 150 mm pipe?
VVR 120 Fluid VVR 120 Fluid MechanicsMechanics
C7 Using the Y and the control volume in the fig. find the mixture flowrate and density if freshwater (ρ1 = 1000 kg/m3) enters section 1 at 50 l/s, while saltwater (ρ2 = 1030 kg/m3) enters section 2 at 25 l/s.