ENH: allow specific restart time for field averaging (issue #282)

- Can currently have a periodic restart, but for simulations with a
  known run-up, it can be useful to have a specific time to restart
  the averaging.

- Note that the restartTime acts as a 'single-shot'. If the
  restartTime is already in the past when a simulation is started, it
  is ignored. If, during a simulation, the restartTime is crossed, it
  will be triggered and then set itself to be ignored in the future.

src/functionObjects/field/fieldAverage/fieldAverage.C

@@ -141,12 +141,24 @@ void Foam::functionObjects::fieldAverage::calcAverages()
         prevTimeIndex_ = currentTimeIndex;
     }
 
+    bool doRestart = false;
     if (periodicRestart_ && currentTime > restartPeriod_*periodIndex_)
     {
-        restart();
+        doRestart = true;
         periodIndex_++;
     }
 
+    if (currentTime >= restartTime_)
+    {
+        doRestart = true;      // Restart is overdue.
+        restartTime_ = GREAT;  // Avoid triggering again
+    }
+
+    if (doRestart)
+    {
+        restart();
+    }
+
     Log
         << type() << " " << name() << " write:" << nl
         << "    Calculating averages" << nl;
@@ -262,6 +274,7 @@ Foam::functionObjects::fieldAverage::fieldAverage
     restartOnOutput_(false),
     periodicRestart_(false),
     restartPeriod_(GREAT),
+    restartTime_(GREAT),
     initialised_(false),
     faItems_(),
     totalIter_(),
@@ -296,6 +309,25 @@ bool Foam::functionObjects::fieldAverage::read(const dictionary& dict)
     if (periodicRestart_)
     {
         dict.lookup("restartPeriod") >> restartPeriod_;
+        Log
+            << "    Restart period " << restartPeriod_
+            << nl << endl;
+    }
+
+    restartTime_ = GREAT;
+    if (dict.readIfPresent("restartTime", restartTime_))
+    {
+        if (restartTime_ < obr_.time().value())
+        {
+            // The restart time is already in the past - ignore
+            restartTime_ = GREAT;
+        }
+        else
+        {
+            Log
+                << "    Restart scheduled at time " << restartTime_
+                << nl << endl;
+        }
     }
 
     readAveragingProperties();

src/functionObjects/field/fieldAverage/fieldAverage.H

@@ -3,7 +3,7 @@
   \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox
    \\    /   O peration     |
     \\  /    A nd           | Copyright (C) 2011-2016 OpenFOAM Foundation
-     \\/     M anipulation  | Copyright (C) 2015 OpenCFD Ltd.
+     \\/     M anipulation  | Copyright (C) 2015-2016 OpenCFD Ltd.
 -------------------------------------------------------------------------------
 License
     This file is part of OpenFOAM.
@@ -106,6 +106,7 @@ Usage
         restartOnOutput | Restart the averaging on output | no | no
         periodicRestart   | Periodically restart the averaging | no | no
         restartPeriod     | Periodic restart period | conditional |
+        restartTime       | One-shot reset of the averaging | no | great
         fields            | list of fields and averaging options | yes |
     \endtable
 
@@ -166,6 +167,9 @@ protected:
         //- Restart period
         scalar restartPeriod_;
 
+        //- Specific restart time
+        scalar restartTime_;
+
         //- Initialised flag
         bool initialised_;