Janis Viba, Vitaly Beresnevich and Martins Irbe
3. Theory and simulation of plane motion of flat plate
To determine coefficient C, numerical simulations in Solid Works for flat plate interaction with air flow were performed [8, 9]. Modeling results for a rectangular flat plate were compared with analytical formula (6). It was shown that approxi-mate value of coefficient C is about 0.5. The estimation of accuracy of formula (6) for C = 0.5 in comparison with simulation results is presented in graphical form in Figure 2.
From the analysis of Figure 2, it can be concluded that interaction force fluctu-ates within certain limits, and value of fluctuation is dependent on angleβ. At smaller angles, the value of fluctuations is also small, but with the increasing ofβ fluctuation of interaction force becomes greater. The relative difference is not very large, and its mean value is about 3.6%.
Therefore, engineering analysis by the proposed method does not require a step by step space-time calculations to find interaction coefficient C. A new
nonstationary flow interaction formula was obtained, and this formula includes the object shape, state, and flow rate direction parameters. Coefficient C is taken as C = 0.5. The efficiency of the proposed method is illustrated by the analysis of flat plate interaction with air flow.
Figure 2.
The accuracy of formula (6) for C = 0.5 as function of angleβ.
Nonstationary regimes of interaction between continuous air flow and rigid body, as shown in [5], mathematically are more complicated. Such kind of interac-tion can be analyzed by numerical simulainterac-tion with space time programming methods [6].
In this chapter, a simplified method for the analysis of interaction between air flow and rigid body is proposed. Method is based on separate consideration of flow-body interaction on pressure and suction (vacuum) sides.
2. Simplified model of air flow interaction with flat plate
To simplify the analysis, optimization and synthesis of wind energetic devices, it is proposed to use approximated model of flow-plate interaction without consid-ering viscous effects of air medium. For this purpose, air flow interaction with the plate is subdivided on two phases: one interaction is on the pressure side and the other one-on the vacuum side (Figure 1).
To analyze air flow-plate interaction, the theorem on change of momentum of mechanical system in differential form, is used [7]. In accordance with this theo-rem, on the side of pressure the following equation can be written:
dmV cosβ¼dNdt, (1)
where dm is an elemental mass of air flow; V is a velocity of air flow;βis the angle between air flow direction and normal to the plate surface L1 (Figure 1); dN is the impact force in the normal direction to the plate surface L2; and t is time.
Elemental mass dm of air flow can be mathematically described by the equation:
dm¼ρVB cosβdt dL (2)
where dL is elemental length of plate’s surface; B is width of the plate (in the case of two-dimensional task B = const); andρis the air density.
By the integration of Eq. (1), extra pressure and force along x-axis can be deter-mined. Mathematically, the pressure distribution can be described by the expressions:
pL1¼V2ρðcosβÞ2 (3) pL2¼V2ρðsinβÞ2 (4)
Figure 1.
Air flow interaction with rectangular flat plate.
Vacuum side of the plate is loaded by constant pressure, which can be deter-mined by formula:
Δp2¼ þV2ρC (5)
where C is constant to be found experimentally or by computer simulation.
By this way, it is possible to find total force applied to the body in the air flow.
For example, projection on x-axis of total force F1applied to the rectangular plate is described by the following equation:
F1x¼ �HBV2ρ CþðcosβÞ3þd�ðsinβÞ3 cosβþd�sinβ
" #
, (6)
where d is the ratio of edges L2/L1; H is section height of the plate in the direction perpendicular to the flow (Figure 1, H¼L1 sin βþL2 cos β).
As follows from the results obtained, an approximate analytical method can be applied to solve air flow and blade interaction problems. Validation of the results of analytical calculations can be performed using computer simulation and experi-ments. Examples will be discussed in the following sections.
3. Theory and simulation of plane motion of flat plate
To determine coefficient C, numerical simulations in Solid Works for flat plate interaction with air flow were performed [8, 9]. Modeling results for a rectangular flat plate were compared with analytical formula (6). It was shown that approxi-mate value of coefficient C is about 0.5. The estimation of accuracy of formula (6) for C = 0.5 in comparison with simulation results is presented in graphical form in Figure 2.
From the analysis of Figure 2, it can be concluded that interaction force fluctu-ates within certain limits, and value of fluctuation is dependent on angleβ. At smaller angles, the value of fluctuations is also small, but with the increasing ofβ fluctuation of interaction force becomes greater. The relative difference is not very large, and its mean value is about 3.6%.
Therefore, engineering analysis by the proposed method does not require a step by step space-time calculations to find interaction coefficient C. A new
nonstationary flow interaction formula was obtained, and this formula includes the object shape, state, and flow rate direction parameters. Coefficient C is taken as C = 0.5. The efficiency of the proposed method is illustrated by the analysis of flat plate interaction with air flow.
Figure 2.
The accuracy of formula (6) for C = 0.5 as function of angleβ.
A two-dimensional (2D) model of translation motion of thin flat plate (thickness d�0) in co-ordinate plane x–y is shown in Figure 3. The model includes linear elastic element with stiffness coefficient c and linear viscous damper with damping coefficient b.
In accordance with methods of classical mechanics [7], relative interaction velocity Vrmust be taken into account:
Vr¼Vþv, (7)
where V is velocity of air flow; and v is velocity of flat plate along x-axis.
In this case, differential equation of plate motion along x-axis can be written in the following form:
m€x¼ �cx�bx_�Aρ h0:5þðcosβÞ2i cosβ
n o
ðVþxÞ_ 2signðVþxÞ,_ (8) where A = LB is a surface area of the plate;ρis the air density;βis plate angle against air flow; and m is mass of the plate.
The renewable energy is generated due to the action of damping forceð�bx_Þ: Therefore, momentary power can be determined by formula
P¼bðxÞ_ 2: (9)
The average power Paduring time t is determined by integration of Eq. (9):
Pa ¼ Ðt
0bðxÞ_ 2dt
t : (10)
By the analysis of Eq. (8), it can be concluded that five parameters can be used to control the efficiency of this system. These parameters are as follows: c, b, A, β, and V.
Mathematical simulation of Eq. (8) was performed with program MathCad assuming the following values of main system’s parameters: A = 0.04 m2(length L = 0.2 m and width B = 0.2 m); V0= 10 m/s;ρ= 1.25 kg/m3(at temperature 10°C).
Results of simulation for control action by angleβ¼2π:5sin 7tð Þare presented in Figures 4 and 5.
Average power Pa(Figure 5) is presented as percentage of maximal power, which can be achieved under the plate’s velocity less than one third of flow velocity.
Figure 3.
Model of thin flat plate to obtain renewable energy from air flow.
Results of simulation for the case of control action on the system by harmonic variation of angleβ(Figures 4 and 5) are as follows:
• stationary regime of motion occurs very quickly, practically within two to three cycles;
• it is possible to synthesize the optimal parameters of the system (for example, stiffness c, area A, frequency and amplitude of action, etc.), which would provide the maximum power within the given limitations;
• further increasing of the efficiency can be achieved by the use of more complex control variation of angleβ(biharmonic, polyharmonic, etc.).
Results of simulation of plate’s motion for the case of control action by velocity V¼V0½2�0:5 sin 10tð Þ� are presented in Figures 6 and 7 (it is assumed that V0 ¼10 m/s).
As it is seen from the analysis of graphs presented (Figures 6 and 7), almost stationary oscillatory regime with maximal average power Pacan be achieved after some cycles of transient process.
Figure 4.
Displacement x as function of time t.
Figure 5.
Average power Paof generator force bx during short transient process (t = 4 s)._
A two-dimensional (2D) model of translation motion of thin flat plate (thickness d�0) in co-ordinate plane x–y is shown in Figure 3. The model includes linear elastic element with stiffness coefficient c and linear viscous damper with damping coefficient b.
In accordance with methods of classical mechanics [7], relative interaction velocity Vrmust be taken into account:
Vr¼Vþv, (7)
where V is velocity of air flow; and v is velocity of flat plate along x-axis.
In this case, differential equation of plate motion along x-axis can be written in the following form:
m€x¼ �cx�bx_�Aρ h0:5þðcosβÞ2i cosβ
n o
ðVþxÞ_ 2signðVþxÞ,_ (8) where A = LB is a surface area of the plate;ρis the air density;βis plate angle against air flow; and m is mass of the plate.
The renewable energy is generated due to the action of damping forceð�bx_Þ: Therefore, momentary power can be determined by formula
P¼bðxÞ_ 2: (9)
The average power Paduring time t is determined by integration of Eq. (9):
Pa ¼ Ðt
0bðxÞ_ 2dt
t : (10)
By the analysis of Eq. (8), it can be concluded that five parameters can be used to control the efficiency of this system. These parameters are as follows: c, b, A, β, and V.
Mathematical simulation of Eq. (8) was performed with program MathCad assuming the following values of main system’s parameters: A = 0.04 m2(length L = 0.2 m and width B = 0.2 m); V0= 10 m/s;ρ= 1.25 kg/m3(at temperature 10°C).
Results of simulation for control action by angleβ¼2π:5sin 7tð Þare presented in Figures 4 and 5.
Average power Pa(Figure 5) is presented as percentage of maximal power, which can be achieved under the plate’s velocity less than one third of flow velocity.
Figure 3.
Model of thin flat plate to obtain renewable energy from air flow.
Results of simulation for the case of control action on the system by harmonic variation of angleβ(Figures 4 and 5) are as follows:
• stationary regime of motion occurs very quickly, practically within two to three cycles;
• it is possible to synthesize the optimal parameters of the system (for example, stiffness c, area A, frequency and amplitude of action, etc.), which would provide the maximum power within the given limitations;
• further increasing of the efficiency can be achieved by the use of more complex control variation of angleβ(biharmonic, polyharmonic, etc.).
Results of simulation of plate’s motion for the case of control action by velocity V¼V0½2�0:5 sin 10tð Þ� are presented in Figures 6 and 7 (it is assumed that V0¼10 m/s).
As it is seen from the analysis of graphs presented (Figures 6 and 7), almost stationary oscillatory regime with maximal average power Pacan be achieved after some cycles of transient process.
Figure 4.
Displacement x as function of time t.
Figure 5.
Average power Paof generator force b_x during short transient process (t = 4 s).
Results of simulation for the case of control action on the system by harmonic variation of velocity V (Figures 6 and 7) are as follows:
• a new opportunity to generate energy by the harmonic variation of flow rate is discovered;
• new variants of control action on the system by the flow rate variation using more complex laws (biharmonic, polyharmonic, etc.) are opened for the further research.