Upgrading Custom Hydraulic Blocks to Use the Isothermal Liquid Domain

If your model contains custom blocks with hydraulic ports, you can rewrite the underlying component source to adapt them to using the isothermal liquid domain.

This change may lead to numerical changes in the block behavior. Using mass flow rate, instead of volumetric flow rate, as the Through variable reduces the potential for small errors in mass conservation to accumulate over time due to the conversion between mass and volumetric quantities, which results in increased accuracy.

To rewrite the component source, follow these steps:

1. Replace the nodes of type foundation.hydraulic.hydraulic with foundation.isothermal_liquid.isothermal_liquid.

2. In the variables section, replace the Through variable q with mdot. q represents volumetric flow rate and has units of volume over time, such as m^3/s. mdot represents mass flow rate and has units of mass over time, such as kg/s.

3. Add an intermediate, rho, to represent fluid density. Use the provided library function to calculate density based on pressure at the port, for blocks with a single fluid port, or based on average port pressure, for blocks with two or more fluid ports. To view the source file of this function, at the MATLAB^® command prompt, type:

   open([matlabroot '/toolbox/physmod/simscape/library/m/+foundation/+isothermal_liquid/mixture_density.ssc'])

4. Rewrite the equations by replacing q with mdot/rho.

For example, consider this custom component, which models a hydraulic linear resistance.

component custom_linear_resistance
% Custom Linear Hydraulic Resistance 
% This block represents a hydraulic resistance where pressure loss
% is directly proportional to flow rate. 
%
% Connections A and B are conserving hydraulic ports associated 
% with the block inlet and outlet, respectively. The block positive 
% direction is from port A to port B. This means that the flow rate is 
% positive if fluid flows from A to B, and the pressure loss is determined
% as p = p_A - p_B.

% Copyright 2005-2023 The MathWorks, Inc.

  nodes
    A = foundation.hydraulic.hydraulic; % A:left
    B = foundation.hydraulic.hydraulic; % B:right
  end

  variables (Access = protected)
    q = { 1e-3 , 'm^3/s' };    % Flow rate
    p = { 0 ,     'Pa'   };    % Pressure differential
  end

  branches
    q : A.q -> B.q;
  end

  parameters
    resistance = { 1, 'GPa/(m^3/s)' }; % Resistance
  end
           
  equations
      % Assertion
      assert(resistance >= 0)
      
      p == A.p - B.p;
      p == resistance * q;
  end
   
end

To adapt this component to use the isothermal liquid domain:

1. Declare nodes A and B as foundation.isothermal_liquid.isothermal_liquid.

2. Under variables, replace q with mdot.

3. Add the rho_avg intermediate, which calculates density based on average port pressure. The density calculation uses the Foundation library function foundation.isothermal_liquid.mixture_density.

4. Rewrite the equation p == resistance * q; by replacing q with mdot/rho_avg.

The new component, custom_linear_resistance_il, now models an isothermal liquid linear resistance.

component custom_linear_resistance_il
% Custom Linear Resistance (IL) : 
% This block represents a hydraulic resistance where pressure loss
% is directly proportional to flow rate. 
%
% Connections A and B are conserving isothermal liquid ports associated 
% with the block inlet and outlet, respectively. The block positive 
% direction is from port A to port B. This means that the flow rate is 
% positive if fluid flows from A to B, and the pressure loss is determined
% as p = p_A - p_B.

% Copyright 2005-2023 The MathWorks, Inc.

  nodes
    A = foundation.isothermal_liquid.isothermal_liquid; % A:left
    B = foundation.isothermal_liquid.isothermal_liquid; % B:right
  end

  variables (Access = protected)
    mdot = { 0.1 , 'kg/s' };    % Mass flow rate
    p = { 0 ,     'Pa'    };    % Pressure differential
  end

  branches
    mdot : A.mdot -> B.mdot;
  end

  parameters
    resistance = { 1, 'GPa/(m^3/s)' }; % Resistance
  end
           
% For logging
intermediates (Access = private)
    rho_avg = foundation.isothermal_liquid.mixture_density((A.p + B.p)/2, ...
        A.bulk_modulus_model, A.air_dissolution_model, A.rho_L_atm, A.beta_L_atm, ...
        A.beta_gain, A.air_fraction, A.rho_g_atm, A.polytropic_index, A.p_atm, ...
        A.p_crit, A.p_min); % Average liquid density
end
  
  equations
      % Assertion
      assert(resistance >= 0)
      
      p == A.p - B.p;
      p == resistance * mdot/rho_avg;
  end
   
  end